In general, conventional image forming apparatus such as copiers and laser printers employing an electrophotographic system or electrostatic recording system have a configuration in which image exposure is performed on a surface of a photosensitive drum to form an electrostatic latent image; the electrostatic latent image formed on the surface of the photosensitive drum is developed by a developing device to form a toner image in a predetermined color, and the toner image is directly transferred on to and fixed on a sheet of recording paper, a sheet of other substrate media or temporarily transferred to an intermediate transfer body and is thereafter transferred on to the recording paper at a time to form an image. The area where the photoreceptor engages and/or interacts with the belt or sheet is referred to as the transfer area. Transfer of the image to the sheet or transfer body should be in precise registration, otherwise it can cause processing interruptions or delays and/or impair the print quality.
In the case of a full-color printing apparatus, there are typically four development units; cyan, magenta, yellow, and black (CMYK). In a “highlight color” printing apparatus, where it desired to print black plus one other predetermined color, a typical arrangement is to have a black development unit and one or more development units, one for each of a selectable set of highlight colors, only one of which would be used at a time. Other types of architecture include “hexachrome,” where there are two additional color development units beyond CMYK, thus providing an extended color gamut for the printer; and arrangements that include a development unit for applying clear toner, or one applying a toner with special properties such as MICR (magnetic ink character recognition) toner.
Examples of typical basic color xerographic architectures are shown in U.S. Pat. Nos. 6,628,909; 7,177,585; and 6,871,037. Variously, the development units could be arranged around a single photoreceptor belt; each development unit could be associated with a single drum photoreceptor, and the drum photoreceptors arranged around a common “intermediate belt” that accumulates the primary-color toner images for transfer to a print sheet; or the drum photoreceptors could each directly transfer their primary-color images to a sheet moving past each photoreceptor. U.S. Pat. No. 6,718,879 and U.S. Patent Application Publication 20010043823 show examples of control systems useful for accurate placement of images in a large color printer.
Contemporary systems assume a constant and smooth motion of the sheet as it travels through the transfer area. Thus, it is assumed that the orientation and lateral position of the sheet as delivered into the transfer area and tacked to the photoreceptor drum or belt remains the same exiting the transfer area. In fact, in order to ensure a constant and smooth transition, many systems release sheet nip assemblies upstream from the transfer area, as soon as the sheet leading edge tacks. Such a nip release can minimize any forces and velocity vibrations from affecting the sheet motion in the transfer area. Additionally, post-transfer transports are designed in a way to minimize the forces and torques on the sheet as its leading edge is acquired after image transfer and the sheet is transported to a fuser. However, particularly in modular print assemblies, sheet transfer between modules can accumulate sheet velocity vector errors. The accumulation of these velocity vector errors can impart large push/pull forces on the sheet. With medium or light-weight substrate media such forces can cause wrinkling, buckling and/or tearing of the sheet. Further, in modular overprint systems where each module prints over a previous module, the push/pull forces can lead to large Image-on-Paper registration and color-to-color registration errors.
Accordingly, it would be desirable to provide a method and system of dynamically registering an image relative to the recording paper, other substrate media or intermediate transfer body in a printing system in order to avoid processing interruptions or delays, poor quality image registration and other shortcomings of the prior art.